This invention relates to solid-state circuit breakers for controlling the supply of power from an AC supply to a load and particularly to such circuit breakers that are suitable for applications in which size and weight must be minimized and reliability must be maximized while closely simulating the functions of conventional electromechanical apparatus.
In Billings U.S. Pat. No. 3,879,652, Apr. 22, 1975, there was disclosed an AC solid-state power controller, which disclosure is incorporated herein by reference. Such apparatus is quite satisfactory for its intended purposes. The denomination of such apparatus as a "power controller" could also be applied to that of the present invention. However, it is instead chosen to refer to the present apparatus as a solid-state "circuit breaker" to convey the suggestion that the apparatus has capability for interruption in response to a manually actuated element or switch, as an electromechanical circuit breaker does, in addition to the internal power control functions therein.
The off/on/reset toggle switch of conventional circuit breakers utilizing magnetic and thermally responsive elements is a desirable feature and affords flexibility and convenience in the testing, maintenance and use of the breaker. Additionally, electromechanical breakers have the quality of power contact, meaning that the closed or on position provides direct conduction of power to the load with negligible impedance and upon interruption there is a clean break imposing an infinite impedance in the load circuit. Such apparatus is also capable of inverse trip time overload protection, that is, it can be made responsive in a shorter time to a higher overload condition than a smaller overload condition. Also, conventional circuit breakers can readily be provided with means for status indication of the contacts. All these functions are desirably to be achieved in a solid-state circuit breaker which avoids problems of mechanical wear, reliability, and bulk.
Among the qualities not provided by conventional type circuit breakers is any means for insuring occurrence of the closing or interruption function in accordance with a predetermined point in the applied power wave form. Such synchronism becomes an important objective in achieving a solid-state circuit breaker because the power switch elements, normally thyristors, are, in contrast with mechanical contacts, peculiarly dependent in their operation and performance on the applied wave shape.
Also, conventional circuit breakers inherently exhibit overload protection within a wide tolerance range which is desirably to be made more precise.
Other qualities of conventional circuit breakers tending to make them unattractive in high reliability situations is that they are affected by ambient temperature, dust, moisture, and the like, and exhibit relatively low physical strength and endurance to shock and vibration.
The general purposes of the apparatus of the present invention are to provide a solid-state circuit breaker with the good qualities of electromechanical circuit breakers while markedly improving on the poorer qualities so that the apparatus is capable of performance with high reliability in a compact and low weight structure making it particularly suitable for use in aerospace, military and oher applications where these performance advantages are desirable.
In accordance with the present invention, a solid-state circuit breaker is provided with circuitry for permitting off/on/reset operations actuated by a mechanical toggle control switch for close simulation of magnetic circuit breaker operation. Locating the control switch so that it opens essentially all of the circuitry except for the static power switch branch, between the source and the load, provides a high degree of fail-safe protection for the apparatus, or a power removal feature. A power continuity feature includes means for zero crossover synchronization both upon power-up and power-down conditions with full sine wave conduction through the static power switch regardless of the instant at which the manual control switch is opened or closed. A circuit lockout feature is used so that on power-up, or turn-on, operation is delayed until the occurrence of an internal power-ready condition. On power-down, or turn-off, there is means for circuit retention subsequent to switch opening and internal power removal in order to accomplish load turn-off at a desired zero crossover point of the load power through the power switch. A power supply is incorporated that does not require stepdown or isolation transformers and improves capability for miniaturization of the overall circuitry and enables design of the circuit for operation at any of a wide range of frequencies, such as 60 Hz to 400 Hz.
The invention is generally an improvement on the "power controllers" such as that of U.S. Pat. No. 3,879,652 in order to achieve additional "circuit breaker" functions.